fix: add metadata to collection resp (#9514)

**What**
- Adds `metadata` to the default response for `/admin/collection` endpoints.
- Fixes tests to expect correct shape and not use objectContaining.

Fixes CC-551

packages/medusa/src/api/admin/collections/query-config.ts

@@ -4,6 +4,7 @@ export const defaultAdminCollectionFields = [
   "handle",
   "created_at",
   "updated_at",
+  "metadata",
 ]
 
 export const retrieveTransformQueryConfig = {